Ever since I've owned my diesel Caddy, I really wished it had a
tachometer. The one option that is available on later model VW diesels
is the "W-terminal" alternator. This alternator has an extra
terminal (labeled "W") that allows an RPM-proportional signal
that can drive a dash-mounted tachometer. It is common on the turbo
diesel models and requires a full gauge cluster swap to have the tach
replace the stock clock. The "W-terminal" also is used to
drive the "upshift" indicator that is apparently used on
certain '82 and later models.

Unfortunately, my original '81 alternator is not the "W" type
and it is still working fine, so I don't really want to replace it,
same with the instrument cluster. The alternator is driven off the
engine by a v-belt, so the readings provided by the tachometer are
probably not really accurate. In any event it would have to be
calibrated in some manner (I guess by a mechanical tach?). So, I
decided it was best to design a fully digital tachometer system to
avoid all these problems.

One of my old engineering projects I worked on at NASA involved using
optical shaft encoders to pick up shaft speed and rotation for use in
position sensors. These encoders use a series of concentric circles of
alternating light and dark marks to detect motion. For speed only, one
set of marks is sufficient.

In the case of the VW diesel, the injector pump and cam shaft are
driven by a toothed belt off the crankshaft. The crank shaft sprocket
is 1/2 the diameter of the rest of the sprockets, so they spin at 1/2
the engine RPM. The crankshaft sprocket is not easy to get to, but the
one on the fuel injector pump is accessible by an existing hole in the
timing belt cover, at least on my A1/1.6D engine. Since the injector
pump sprocket is turning 1/2 speed of the engine, it is necessary to
have the pickup wheel make two on-off cycles per revolution.

I fabricated the pickup wheel from a sheet of stainless steel sheet
metal. I sized it to fit inside the sprocket on the fuel injection
pump, this made the outer diameter 118mm with a 43mm hole cut out of
the center. I masked off two opposite quadrants and spray painted the
remainder with flat black enamel paint. The wheel is attached to the
sprocket on the fuel injection pump with a flexible adhesive. In the
above picture, you can see one black (non-reflective) quadrant just
below and to the left of the retaining nut on the injection pump
pulley. The two reflective quadrants are visible just under the A/C
hose that runs across the middle of the picture.

Conveniently, there is a rubber plug in the timing belt cover at just
the right place to allow the photo-sensor to "look at" the
bottom of the injector pump sprocket. I trimmed the mounting tabs on
the sensor until it was 3-5mm away from the surface of the pickup
wheel. I cut the center of the plug and then glued the sensor into the
plug. The entire plug/sensor may easily be removed from the timing belt
cover if needed.

Once the sensor was in place, I created a mounting hole for the digital
meter in the dash. To the left is the stock emergency flasher switch,
to the right is the fog light switch I added. I may end up tilting the
display upwards a bit for better visibility. Here, you can see it
registering 779 RPM at idle. The nice thing about this digital tach
setup is that it is precise and no calibration is required, the
accuracy is built into the design. This tachometer also reads out to
the individual RPM, not just 100's like run-of-the-mill digital tachs.

The wiring for this design turned out to be deceptively simple. The
photosensor has 4 terminals, and they were different than the Omron
data book I had. The book describes them a Anode, K(c)athode, Emitter,
and Collector, but the sensor had +,-,L and Out.
After some trial and error, I found that the internal LED was connected
internally to the + terminal and I had to add a current-limiting
resistor between the L and - terminals. I used a 620 ohm
resistor which limits the current to about 25mA. Likewise, the Out
terminal is an open collector output and by tying it to +
through a load resistor (620 ohms) gave me a nice 0-12V swing on the
output and again about 25mA of current. I soldered the resistors
directly to the terminals on the sensor, then ran 3 wires for +12V,
ground and output through the firewall to the panel meter. I spliced
into the radio power for +12V and ground (switched w/ ignition) and
hooked the power and signal to the meter. I used 1/2 watt resistors for
this application, and selected a value to limit the current to above
value. From the photosensor data sheet, resistance values up to about
1K ohms should produce acceptable current values, don't go any lower
than 600 ohms, though.

Below are the parts I used in this project. I ordered the tachometer
directly from Acculex (see link below) and the photo-microsensor was
ordered from Digikey along with a
very informative Omron Application Guide (highly
recommended). The pickup wheel material was purchased at a local
hardware store and the other parts were out of my well stocked parts
box (but are readily available):

Basically, you are looking for a device that will turn on and off with
either a change in reflectivity or by physically interrupting the
optical path with some sort of slotted wheel. Usually, this consists of
a light emitting diode (LED) that provides the illumination, and a
phototransistor that senses the presence or absence of that
illumination, To avoid problems with stray light sources, infrared is
preferred over visible light, and the sensitivity or the photo
transistor should be closely matched to the output of the LED. LEDs
tend to have a rather narrow output wavelength and also beam width
(think of them as tiny lasers), It is important to collect and focus
the light out of the source, onto the target and then back to the
sensor. While it is possible to make your own photo-sensor, I felt for
$12 (actually only $8.50 when I bought mine) my time was better spent
on the rest of the system than fooling with more discrete components,
etc.

A key feature of these optical sensors is the use of infrared LEDs and
photo transistors. This both prevents interference from stray external
light sources and by having the two components match for a specific
frequency of radiation, they are even more immune to interference.
Because of this, you won't see any visible light coming from the LED
when its operating.

After designing and installing the digital tachometer described above,
and publishing this project on the world wide web, I had a few readers
ask about applying this design to an analog tachometer. In order to see
what changes are required for an analog tachometer, it is necessary to
understand a bit about how they work and compare that to how the
digital tachometer (or more correctly rate meter) that I already had
works.

Any rate meter simply measures the rate at which some event occurs.
Usually this is done by counting the events (contact closures,
electrical pulses, etc.) for a given period of time (known as the
integration interval) and then simply dividing the number of events by
the time to get a rate. In my case, I had a rate of 1 pulse per engine
revolution and by using a rate meter calibrated in events per minute, I
could display revolutions per minute.

In a spark ignition engine, tachometers typically use a somewhat
different mechanism to obtain engine speed. Since the ignition is
firing the spark plugs, there is a handy signal source present off of
the ignition coil primary (in older vehicles at least) that is in the
form of a square wave (0-12V). If you know how many cylinders and
assuming a 4-stroke engine, which takes two revolutions to complete a
full cycle, you can calculate the relationship of ignition pulses to
revolutions as follows:

PulsesPerRevolution = NumberOfCylinders / 2

After market tachometers are often equipped with a selector switch on
the back to choose 4-6-8 cylinder operation. This simply scales the
input based upon the above formula. So, in a typical 4-cylinder VW
gasoline engine, you'll have 2 ignition pulses per revolution. So the
first modification to my digital tachometer circuit is to up the input
pulse rate from 1 to 2 (or more) pulses per revolution.

Unlike the digital rate meter described above, the analog tachometer is
usually built up out of an electro-mechanical current meter and some
sort of input conditioning circuitry. Instead of integrating the input
signal over discrete time intervals, the mass of the meter movement
itself is used to continuously integrate the input pulses. Thus, if the
meter takes 10mA to swing full scale, if you were to switch the current
between 0 and 10mA at a fast enough rate, the meter needle would read
out the average of the on and off cycles. In order for this to work, it
is necessary to condition the input signal to provide a repeatable
output signal given an possibly varying input signal.

The input circuity is used to send calibrated pulses to the meter,
usually done with what is known as a one-shot timer. For purposes of
discussion, assume the one-shot timer send out a 1mS wide pulse for
every input pulse (of varying duration) and the current is calibrated
to the 10mA needed by the meter. If the input pulses come at the rate
of 10/second (or every 100 mS), the meter will "see" an
average current of 1/100 * 10mA or 1% of full scale. If the input speed
is upped to 200/second (5mS), then the meter will "see" 1/5 *
10mA or 20% of full scale. Upping the input frequency to 1000/sec will
give a full 100% reading on the meter.

However, I'm not very interested in designing a tachometer from scratch
(been there, done that, bought the T-shirt) I just want design a
circuit to drive one. One subtle and not so obvious difference between
a digital rate meter and an analog tachometer is in the input. Since
the tachometer is designed to be driven off of the ignition primary of
the engine, it has a very strong input signal (i.e. low impedance) and
the load of the tachometer on the ignition is trivial. A digital rate
meter on the other hand is most likely designed to operate with high
impedance signals and thus probably has a suitably high input impedance
(10 MOhms is typical). So, simply hooking up the output of a photo
sensor to any run of the mill automotive tachometer may not be
successful.

In my case, I was originally using an internally amplified photo sensor
and while it easily drove my digital rate meter, it had some trouble
with the large tuneup tachometer I had at home. The tach would
invariable read about 2/3 the value of the digital meter. It turns out
the relatively low input impedance of the analog tach was dragging down
the output of the sensor, which caused the RPM reading to fall. While I
was only operating the photo sensor at about 1/2 its rated current and
probably could have pushed it a bit harder, I felt that design was too
marginal for comfort, possibly shortening the life of the component in
the elevated temperatures under the hood.

So, I set about making more extensive modifications to the design as
described below:

This readout module is not back-lit, making it invisible in the dark, I
could not find a good way to backlight the display.

It is a bit pricey, over 80% of the cost of this project

Anyway, to accommodate an analog tachometer, designed to operate off a
4-cylinder gasoline engine ignition system, a very simple change is
required. By simply laying out 4 dark and 4 light segments (instead of
2) you'll make a wheel that produces 2 on-off cycles per revolution,
exactly duplicating a 4-cylinder spark ignition engine. That's the easy
part.

If you are lucky, and have a good high impedance analog tachometer, you
might be able to construct a circuit as shown below and have it work.
Note the use of the amplified EE-SB5V photo sensor. Dropping the values
of the two resistors to about 500 ohms would push the input and output
currents up and hopefully drive a decent tachometer.

For a more universal solution, I decided to take a slightly different
approach. Since the added cost of the internal amplifier in the EE-SB5V
was both substantial and inadequate for this application, I instead
chose the lower cost EE-SB5 (non-amplified sensor). Below is a sketch
of what the revised circuit looks like:

(click to download a larger version)

The resistor on the EE-SB5 LED is sized to provide between 20 and 50 mA
(max.) to the sensor, I chose 620 ohms, since I had one in my parts
drawer.

The resistor on the EE-SB5 photo-transistor is sized to provide ~20mA
load on the output 620 ohms is about right for 20mA on both sides and
power rating of about 1/4 to 1/2 watt (i.e. I^2*R = 0.020 * 0.020 * 620
= 0.248W)

However, in conducting some experiments with the non-amplified EE-SB5
detectors, I found that there wasn't enough current gain (known as beta)
in the photo transistor to drive anywhere near 20mA on the output, so I
had to increase the resistor to about 33K ohms to get a decent voltage
swing

Transistors are essentially current amplifiers, in this case, the input
current is due to infrared photons from the LED hitting the exposed
base of the transistor, knocking electrons free, which creates the
current flow; in this case I was seeing perhaps 0.35mA flow, so 33K
resistance gives about a 12V signal.

Note: for a given amount of current, the voltage drop is
related to the resistance as in V=I*R

Unfortunately, this rather high output resistance created another
problem when trying to drive an electro-mechanical tachometer, with
perhaps 10-100K ohms input impedance:

This extra loading (when compared to a 10 Mohm digital meter) is enough
to pull the output signal down next to 0.

So, the solution was to add a second stage of current gain similar to
the EE-SB5V, but at less cost and without adding it in the sensor
itself. So, I returned to another old design solution and the venerable
555 timer. You'll notice it is the same chip used in the previous
analog tachometer circuit, but in this case I have it wired up as a
Schmitt trigger to both clean and boost the signal up into the 100mA
range.

The following web page has an excellent tutorial on 555 timer operation

Adding this additional circuitry requires the addition of a small
electronics box to the system, but this box will provide a nice place
to connect things up at. The bipolar Schmitt trigger can drive about
200mA loads and has no trouble with the large shop tachometer I'm
testing with. The signal is a nice clean square wave and seems to work
at both the lower and upper RPM ranges. And this circuit allows an
un-altered analog tachometer off of a gasoline (spark ignition) engine
to work off of a diesel engine.

A number of these tachometer drive kits have been successfully adapted
to work on other than VW diesel vehicles. Among those applications have
been on vehicles converted to electiric operation and to marine and
stationary diesel engine applications. All those applications are
similar in that there is no ignition type signal available to drive the
tachometer. As far as the tachometer signal generator is concerned, it
does not care what sort of moving equipment it is connected to. If it
rotates, it can generate a signal proporional to the RPM and feed that
to drive an analog tachometer. In these applications, a custom pickup
wheel and sensor mounting bracket would of course need to be fabricated
by the end user.

Thoroughly clean the inside of the injector pump sprocket, you may want
to spray a bit of flat black paint on the two sprocket alignment holes
if they are shiny

Be sure to line up the two holes with the sprocket alignment holes
before gluing in place

For adhesive, we use "Automotive Goop" or a good high
temperature, oil-resistance silicone adhesive-sealant or something like
JB Weld will work as well.

Clamp the pickup wheel in place until the adhesive cures

Note that these photos are of the prototype pickup wheel. The current
production model has 3 holes in it to allow insertion of the pump
timing lock tool and to access the lower pump retaining nut and bolt
for timing adjustments. So you should be sure and align the holes in
the pickup wheel with those holes in the sprocket.

Pop out the small rubber plug and install the optical sensor to the
inside of the belt cover

This applies to the Volkswage A1 diesel engines, some later models may
require the drilling of a 5/8"/16mm hole over the injector pump
sprocket, roughly over the center of the spokes.

IMPORTANT:

Check the clearance between the pickup wheel and sensor, ideal
clearance is approx. 5mm.

With the cover installed:

Measure from the outside of the cover in to the pickup wheel (Picture
2a)

Measure the length of the photo sensor from its mounting point (Picture
2b)

The difference should be 1/8" to 1/4"

If not adjust with the sensor or the timing belt cover with washers

If placed too close to the wheel, the sensor will be damaged

If its too far away, the sensor may not get a good signal

Make sure the cover is securely fastened and doesn't vibrate
excessively when the engine is running

If you don't have all 4 mounting bolts installed, replace them before
installing the sensor

You can adjust the photo sensor clearance by moving washers on it or
the timing belt cover

If needed, you can also bend out the face of the timing belt cover for
more space.

Once the depth is set, install the sensor and tighten down the
retaining nut (Picture 2c)

Note that the old style pickup wheel design is shown in this image, the
new version has holes to allow for locking the pump sprocket in place
for timing as well as an access hole for the lower pump mounting bolt, click here for an image
of that new design part. When installing the new
wheel, be sure to align the two full holes with the alignment holes in
the sprocket.

Take care to orient the sensor so its long axis is aligned
circumferential with the rotation of the pickup wheel

Re-install the timing belt cover

If in doubt on the sensor clearance, crank the engine over manually and
check the sensor and pickup wheel for signs of contact

Don't start the engine until you are positive the sensor won't contact
the pickup wheel

And most importantly, BE SURE THE BELT COVER IS SECURE, make sure all
the bolts and spacers are in place and tight before starting the engine
and driving. You do not want the cover to come loose while driving as
it or one of the mounting bolts can easliy come loose and damage the
timing belt.

Route the wire from the module to the sensor and attach the supplied
connector

See: Picture 3

Connect the +12V supply wire to the fuel cutoff solenoid valve on the
injector pump (blue crimp-on connector just visible in lower center of
image) and connect the ground wire to a suitable ground point, I used
one of the bolts that attach the throttle cable bracket to the pump.

See: Picture 4

You may want to add an in-line fuse for the power line of 1/2 to 1 amp.

Also, be sure you have a clean power source, should have no glitches
(if so, consider adding a noise filtering capacitor of a few uF) and be
sure the voltage is no higher than about 13 volts. Sometimes when
alternator regulators start to fail, they can put out very high
charging voltages (14 volts or higher) and this will damage the IC in
the signal generator box and it will also kill your battery in short
order.

Find a suitable mounting location for the electronics module; the
passenger fender wall is usually available, I had an un-used bracket on
the fender wall.

See: Picture 5

Be sure to mount the electronics box in an area where it will be
protected from water and also make sure to either seal it up water
tight (with silicone sealant) or mount it with the wires downward so
any water that might get into the box will drain out.

Route the remaining wire through the firewall (the hood release grommet
is good for this) and under the dash

This wire supplied without connectors to allow passage through the
firewall.

If you do not use or have an "upshift" light and/or an
RPM-sensitive oil pressure indicator (such as on the A2s) then another
connection option is to remove the wire from the "W terminal"
on the alternator and connect the green wire from the tach generator
box and you are done. That signal is wired back to the tach input wre
on the factory cluster.

For use with non-OEM tachometers, the following wires are available:

Black wire is ground

If not used, wrap this wire in tape to keep it from shorting something
else out.

Red wire is switched 12V if
soldered to the "+" terminal on the circuit board

If not used, wrap this wire in tape to keep it from shorting out.

The yellow or white
wire is unused

If using an aftermarket analog tach, you'll probably be connecting it
to power and ground, so just run all the sender wires up to the
terminals on the tach and it should be ready to go (you may need to
supply a dash light wire for the gauge illumination)

For testing, you may add an LED as shown on the schematic, to check
that the system is working properly:

The LED will flash in time to the engine revolutions, you should see
about 25-30 flashes per second at idle.

Assuming your tachometer has not been modified, the system should need
no calibration.

I found on mine, the needle was pointing about 30° below the
"0" mark, so I had to pull the needle off and carefully
reposition it to point to "0" at 0 RPM, I'm not sure why my
needle was off like this.

After a year or so of use, you may notice the tachometer needle start
to jump a bit at higher RPMs. If this happens, remove the timing belt
cover and clean the face of the optical sensor. Its infrared optics are
fairly tolerant of dirt, but if it gets very dirty it loses sensitivity
and high response speed. Use a water-based window cleaner and soft rag
to clean it.

After a few years of use, I found a totally unexpected benefit of this
tachometer design. I decided to install a cruise control on my
VW diesel and guess what, it needs an engine RPM signal to operate -
BINGO!

If you currently have the two gauge cluster with the analog clock and
you are planning to use an in-dash tachometer in your VW diesel, you
have a few options:

The first is to locate a VW diesel with the real diesel tachometer that
takes its input from the "W" terminal on the alternator and
use that (this assumes you have an alternator with a "W"
terminal).

Most commonly found on the Jetta turbo diesel models.

The "W" terminal is also used to drive the
"Upshift" indicator

Another tachometer option is to use a VW gas tachometer and the proper
alternator, i.e. one with the"W" terminal, and modify the
tachometer as described here.

This technique basically recalibrates the tachometer to handle the
higher output frequency of the alternator W-terminal signal

Here is a copy of the old writeup that was passed on to me by the
author, Mike Musick,
and is available here, although the
information has not been verified and some folks have reported
difficulties getting the modified tachometer to work properly following
these instructions.

So, your first step is to locate a suitable tachometer to use for the
conversion. It is best to obtain an entire instrument cluster if
possible. They can often be found on eBay.
For an A1/Mk1 VW, use one from from an '84 GTi model. For other model
VWs, you would want to use one from a similar model to ensure proper
fitment in the in-dash instrument cluster and for other makes of
vehicles, find a compatible model vehicle to obtain a tachometer from.
Other applications are for vehicles which have been converted from
gasoline to diesel engines and even for vehicles converted to for
example electric drive.

Disassemble the cluster by removing the lamps (save them for spares)
and then the flexible circuit board. Remove the tachometer and the fuel
and water temp gauges that go with it. One advantage of the getting the
entire cluster is that you get a new housing without the hole drilled
to access the analog clock stem. Another advantage is that you are
getting a whole boat load of spare parts, light bulbs, odometer gears,
etc.

Here's a good writeup on removing the instrument cluster if you need
some pointers:

- I reach up underneath and unscrew the speedo cable (assuming you have
a screw-on connection)

- Don't forget the pull-out headlight switch knob, there is a button
underneath the switch to release the knob and shaft (I like to insert
the shaft back in and gently push the switch to the off position w/o
pushing the shaft back into the switch)

- Then if the cluster won't come out easily, some trimming of the back
opening of the dash cutout can make it much easier. My '82 came out
very easy, but my '81 had a lot of flashing left from the molding
process and I had to trim the opening a fair amount to get it out (none
of the trimming is visible with the cluster in place).

In any event, you'll probably want to use your old speedometer, the old
wiring and replace the clock side of things with the tachometer. The
fuel and water temp gauges may be located differently, but should still
work. The only modification needed is to carefully trim back the
"tachometer" "W-terminal" connection from the tach,
tape it off then connect a separate wire to the terminal on the tach
which in turn gets connected to the tachometer pickup signal.

Note: For those who are curious, I stuck an oscilloscope on the
"W-terminal" and found that it delivers a fairly decent
square wave signal, 0-12V or so, at about 25 pulses/revolution. This is
an approximate value as I also had my accurate optical signal on the
second channel and noticed the "W" signal was not locked to
the accurate signal. If you connect it to the gas tachometer, you'll
get abnormally high RPM readings, I saw about 4-5000 RPM at idle. But
this is a good way to test if your gas tachometer is working.

Shown below is a VW A1 diesel instrument cluster. Other models may look
different but the basic idea is to pull the cluster out, remove the
clock and install the tachometer in its place. As far as the wiring
changes, compare the clock and tachometer wiring and identify the power
and ground connections on both. Then the extra wire that the tachometer
has is the one that you replace with the output of the tachometer
signal generator. Now on to the A1 details...

(1)

(2)

(3)

(4)

(5)

(6)

(7)

Remove the stock diesel instrument cluster

See: Picture 1

The green arrow points to the original clock, this gets removed to
install the tachometer

The red circle is around the clock's constant 12 volt power connection,
this is not used with the tachometer, it uses switched power so it
turns off when the engine is off.

The blue circle is likely the clock's ground connection, this is also
not used with the tachometer

The yellow oval is around the area where the cluster wiring plug mates
with the cluster to bring in all the various signals and voltages to
the cluster. So power for the tachometer as well as the W-terminal
signal from the alternator (if factory installed) comes in via this
connector.

Don't forget to transfer the diesel indicator light legend to the new
bezel

See: Picture 4

Install the gas tach and the diesel flexible circuit, then carefully
trim the mylar material back from the stock "tachometer"
connection, tape it off to prevent a short circuit, then connect the
new wire (green in this picture) to the tachometer "B"
terminal (circled in green).

Note: As pictured, I connected wires to the stock
"W-terminal" (yellow wire - yellow circle) and the clock's
+12V circuit (the other green wire - red circle) for conducting some
experiments. Simply wrapping the contact in electrical tape is
sufficient as shown in the light blue circle).

So what functions did I use the old W-terminal and clock 12 volt
connections for? Well, I used the W-terminal signal to determine the
relative frequency of the W-terminal pulses vs. the ones from the
tachometer signal generator as noted just above the photos in this
section. And the clock power? Well, I used it to supply power to the
2" round VDO clock I put in a center console. Instead of
reinventing the constant 12 volt clock power signal, I merely used what
was on the back of the cluster to feed the new clock.

Obviously, you don't need to do this unless you also have plans for
those signals. If not needed, simply wrap those circuit ends in
electrical tape to prevent an accidental short circuit and push them
out of the way.

See: Picture 5

Once everything is back together, slip the cluster back into the dash
and re-install the fasteners.

See: Picture 6

Start the engine and check to see if everything is working.

You'll notice I swapped to the GTi speedometer/odometer, since my old
diesel odometer basically fell apart in my hand when I took it out. I
tried repairing the broken drive gear, but the plastic frame was
cracked where the gear that connects the trip meter and odometer was
and it fell out. I did take care to set the GTi odometer to the same
mileage as my old odometer (and I'll keep it around for backup). I
figure I added 30MPH to my vehicles top speed, too, instead of the
meager 90MPH top end, I now have 120!

See: Picture 7

NOTE:

You can follow the above proceedure to swap a tachometer for a clock in
a gas engined VW as well. Just ignore all the wiring changes I had to
do for the diesel conversion. Also if you are swapping a diesel tach
into a diesel cluster, then again, you don't have to make any wiring
changes, either, since the W-terminal RPM signal is already present at
the tachometer. Its only when swapping a gas tach into a diesel that
you need to connect the RPM signal generator output to the tachometer
input.

Below are the parts I used in this project. I ordered the tachometer
directly from Acculex (see link below) and the photo-microsensor was
ordered from Digikey along with a
very informative Omron Application Guide (highly
recommended). The pickup wheel material was purchased at a local
hardware store and the other parts were out of my well stocked parts
box (but are readily available):

If you purchased the parts kit, you'll need to assemble and test the
electronics module.

Follow the circuit schematic above for connections and also you can use
the parts layout information that is included with the parts kit. You
may notice some minor changes between the schematic and the parts
layout diagram and photo below. The schematic was drawn up based on
laying parts out so they look good on the schematic. The parts layout
and photo below are done in a way that makes the wiring and soldering
easier with the PCB that is supplied with the parts kit. SO if you want
to follolw the schematic to a "tee", feel free to do so. If
you want to follow the parts diagram and photo, feel free to do so.
There are notes in the instructions below mentioning where differences
may be found:

There are two different types of PC boards used, depending on
availability.

One is simply a grid of pads:

First, start by placing the 555 timer chip in the center of the printed
circuit board.

May have a notch on the top side or a mark of sorts for the #1 or #2
pin, depending on the part mfg.

Then, run a bare power wire up the #8 pin side of the board and bare
ground wire up the #1 pin side.

Then insert all the discrete components as desired, arranging them to
end up in the hole next to where they will terminate.

The other is set up for DIP (dual in-line package) chips like the 555
IC and has more connections made

First, place the 555 timer and components per the above parts layout

Note the above photo includes the optional diagnostic LED (green
device) and the blue bypass capacitor (optional, use ~1uF tanatlum
capacitor if desired) at the bottom center.

Don't forget the jumper wire between the #2 and #6 pins:

In the photo below, the jumper is on the under side of the PCB, and not
visible.

If you want to place that jumper wire on top of the 555 along with the
other wires, feel free to do so.

Arrange the parts so the opto-sensor wire will come into the top of the
circuit board, the power and tachometer signal goes out the bottom.

It makes no difference which side of the opto sensor LED that the
resistor goes on, it'll work exactly the same between power and the LED
or between the LED and ground.

So choose the side that results in the cleanest installation.

Solder all the components, including the 5 jumper wires to power and
ground as needed.

Solder the opto-sensor to the short end of the 4-wire connector,
trimming it to is sits as close as possible to the end of the
connector, too long and it'll rub on the pickup wheel.

We find that connecting the Collector to pin 1, Kathode to pin 2, Anode
to pin 3 and Emitter to pin 4 works well.

Be sure to test fit the connector to make sure the sensor is about 5mm
(a bit less than 1/4") from the pickup wheel

Solder the short 4-wire cable from the long end of the connector and
run it into the box, I drill a hole in one end of the box for the wire
to enter, use a simple end to end connection scheme:

We use pin 1 = black, pin 2 = red, pin 3 = yellow, pin 4 = green.

Solder the long wire to power (red), ground (black), and tachometer
signal (green) and run it out of the box.

If desired the 2-conductor wire can be hooked to power and ground.

It is handy to connect to power and ground at the fuel cutoff solenoid
on the injector pump.

Otherwise, run power and ground in on the long wire from the dash.

Alternately, for aftermarket tachometers, the power and ground from the
long cable can be used to power the tachometer.

If you find your charging system has a lot of electrical noise and that
affects the tachometer display (like the needle jumping around too
much), then feel free to add the bypass capacitor as shown. You can get
one at most electronics stores.

For testing, I use a top from a tin can, painted half in flat black and
chuck it up in a cordless drill to serve and a signal source for
testing the circuit.

The circuit has built-in high pass filter to clean up in low level
input signal.

It therefore won't respond to static on/off type signals.

You may be able to use a shiny piece of metal, like the top of the
electronics box, to generate a signal by rapidly moving the sensor on
and off the metal.

The cutoff frequency is around 500 RPM (1000 pulses/minute) so the
input needs to be pretty fast to make it past the filter.

My cordless drill only runs up to around 350 RPM, so I made the test
wheel with 4 pulses per rev, which will "trick" the circuit
into seeing twice the RPM.

If the LED blinks with an input signal, you are set, it works!

If not, time to troubleshoot a bit...

Check all the power and ground connections.

Check the pin 2-6 connection, it should be 1/2 the input voltage, if
not, check all the connections to pins 2 and 6, there are a few.

Check the Emitter (E) output of the opto-sensor before the 0.01 uF
capacitor. It should swing nearly from 0V to 12V as the sensor moves
from shiny to dark.

This input will respond to static light and dark signals, it is before
the filtering 0.01 uF capacitor.

If not, check all the inputs to the sensor.

There should be about a 12V drop across the resistor on the LED part of
the opto sensor. If no drop is observed across this resistor, there is
no current in the LED so the photo transistor won't be getting any
signal.

While I usually pot the opto sensor legs to the connector shell, don't
do so before you are 100% sure its working.

If all these tests fail, double check all the components and solder
connections.

Check all the 555 pins, make sure the power pins have power, the ground
pins are at ground and the input pins (2 and 6) are at 1/2 the supply
voltage.

If it all looks good, most likely the 555 timer has been damaged.

You can get a new one at any Radio Shack (p/n 276-1723)

Note that even a brief power reversal (i.e. +12V to black, ground to
red) will blow out the 555 timer IC

Normal current draw is about 30mA, but if the 555 is dead, you'll only
see about 20mA current which is what the opto-sensor pulls.

If you want the LED to be visible, drill a hole for it in the box lid,
install the circuit board and top and once its all working, apply some
clear silicone sealant to the lid, LED and where the wires enter the
box.